1. Field of the Invention
This invention relates to apparatus capable of producing highly viscous slurries in industrial processes, as well as withdrawing and transferring such slurries once formed. More particularly, the invention relates to the thickening, withdrawal and transfer of slurries that are so viscous that they cannot be removed from a vessel simply by draining or even by conventional pumping techniques.
2. Discussion of Prior Art
The present invention is described in the following with particular reference to the treatment of “red mud” which is an aqueous mineral slurry produced during the extraction of alumina from bauxite by the Bayer process. However, such description is just by way of illustration and the present invention may be used to thicken, withdraw and transfer slurries and muds of various kinds, particularly, although not exclusively, those having clay-sized particles and yield pseudo-plastic properties produced by any industrial process. It should also be noted that, in the following description, the term “mud” is used to mean the same as “slurry”.
During operation of the Bayer process, there are various stages in which red mud is introduced into a vessel and treated for a variety of procedures, such as clarification, washing and thickening of the mud. During such procedures, the mud thickens (i.e. the slurry is separated in a higher solids content fraction) towards the bottom of the vessel to form a thickened bed below a clarified liquor, and the mud is normally displaced or “activated” within the bed by means of a rotating rake or set of arms. Such activation can further increase the thickening of the mud at the lower end of the vessel so that, at the very bottom of the vessel, a highly viscous mud can be formed that is extremely difficult to remove from the vessel on a continuous or intermittent basis. In some parts of the vessel, the thick and highly viscous bed of mud may become stagnant or inactive, making it even more viscous and difficult to extract.
The treatment of red mud in this way is shown, for example, in U.S. Pat. No. 4,830,507 which was issued on May 16, 1989 to Peter F. Bagatto, et al. and is assigned to Alcan International Limited, and also in U.S. Pat. No. 5,080,803 which was issued on Jan. 14, 1992 to Peter F. Bagatto et al. and is assigned to the same assignee.
It has also been observed that when a highly viscous bed of mud forms towards the lower end of the vessel and this mud is being extracted from or near the bottom section of the vessel using a suction pump or similar device, a preferential path of lower viscosity mud (or diluted mud from an upper section) tends to form within the highly viscous bed of mud, leaving higher solids concentration mud un-extracted and stagnant. The phenomenon is informally called “rat-holing” or “doughnut formation” and is undesirable.
The consequence of this situation is that, although some gravity settling vessels can produce highly viscous mud, the resulting mud of high solids content becomes partly diluted by the creation of a preferential path of extraction, so that what is actually extracted is a more dilute mud. This dilution phenomenon induces instability with respect to the concentration and viscosity of the mud being extracted from that vessel and hence introduces severe control difficulties.
The problem of removing highly viscous slurries or muds from vessels of this kind is specifically addressed in U.S. Pat. No. 6,340,033 which was issued on Jan. 22, 2002 to Ronald Paradis et al. and is assigned to Alcan International Limited. The solution to the problem described in this patent involves using a pump or impeller to withdraw slurry from a vessel, subjecting it to high shear, and returning it to the vessel at a point somewhat displaced from the point of slurry withdrawal. The high shear applied to the slurry reduces the viscosity of the slurry (which is referred to as shear-thinning) and thus creates a supply of mud of reduced viscosity. The mud of reduced viscosity, upon re-entry into the vessel, creates a stream of mud within the vessel that entraps particles or clumps of slurry of higher viscosity that are thereby removed from the vessel, recirculated and themselves reduced in viscosity. During the recirculation process, some of the slurry of reduced viscosity is removed and transferred to a different location, thereby continuously withdrawing mud from the system.
While this is an effective solution to the problem of removing slurry of high viscosity from a vessel, it has the disadvantage that the slurry thus removed is necessarily of somewhat reduced viscosity and has to be allowed to settle and stand if a higher viscosity material is required. In many instances, higher viscosity slurry is desirable because it has many of the properties of a solid. Thus, it is at least partially self-supporting when dumped at a land-fill site or other location and can therefore be stacked at a greater height than slurry of low viscosity which tends to flow and dissipate when dumped. Slurry of high viscosity can also be transported on an open conveyor belt, an open truck, or the like, and there is always the option of subjecting it to high shear by means of a pump or impeller, when desired, so that it can be pumped through pipes to another location. Moreover, U.S. Pat. No. 6,340,033 does not fully address the “rat-holing” issue discussed earlier when the mud becomes very thick.
European Patent Publication EP 0 019 538 A1 issued in the name of Alsthom-Atlantique SA, uses a spiral-shaped element to assist the removal of slurry from a tank. The spiral-shaped element is positioned below the tank and acts to remove slurry exiting the interior of the tank through a narrow central opening. The element is largely confined within a closely-surrounding tube or cylinder and only its distal end is aligned with the central opening of the tank. Such an arrangement is likely to be of little use for removing slurry of very high viscosity because such slurry would not flow easily through the narrow central opening provided in the bottom wall of the tank.
Accordingly, it would be advantageous to provide equipment that can generate, on a consistent basis, a highly viscous mud having a high solids concentration, minimizing any internal dilution due to preferential path within the thick mud bed, and without substantially varying the viscosity of the slurry during the removal process.